1. Field of the Invention
The present invention relates to beam splitters and diagnostic systems for high energy laser systems.
2. Description of Related Art
There is generally a need to monitor beam characteristics such as pulse energy, pulse duration, and beam profile for applications using high energy laser systems. This can be done by taking a weak sample of the beam using a beam splitter or by monitoring the low level transmission of a high-reflectivity mirror coating. Optical coatings for a beam splitter can be designed to reflect only a small percentage of the high energy beam or, for a mirror, to transmit only a small percentage. Both of these approaches can be problematic, however, if a very stable, calibrated sample is required for the purpose of monitoring the energy or power in the main beam. The exact reflectivity of high damage threshold dielectric optical coatings can often be a function of environmental conditions such as temperature and humidity and can be very angle sensitive. For example, a high-quality mirror coating might have a reflectivity of 99.5%. If, due to environmental conditions, damage, or even heating from the high power laser beam, the reflectivity drops to 99.4%, it would still be a very good mirror. However, if the optical control system relies on the 0.5% transmitted beam to determine the energy in the main beam, then this 0.1% change will cause an unacceptably large 20% calibration error. A similar argument can be made against the use of a weak reflected beam from an anti-reflective (A/R) coating.
In order to avoid the pitfalls of calibrated transmission or reflection from optical coatings, the laser and beam delivery systems often use only uncoated optical surfaces to sample the high power beam. The reflectivity from these surfaces is determined by the index of refraction of the optical substrate (glass) at the laser wavelength, the angle of incidence, and the beam polarization. The first two parameters are easily controlled in many laser system designs. In some systems, the polarization of the laser beam can be quite stable. However, even small changes in polarization can have a large impact on the amount of power split out of the main beam in these systems.
FIG. 1 shows a prior art beam splitter using a fused-silica optical wedge 351 oriented with an incidence angle of 45 degrees with a P-polarized beam (E-field vector lies in plane of incidence). For optical wedge 351, the P-polarized reflectivity is only 0.6% from each surface of the wedge (called Fresnel reflectivity) for a total transmission of the P-polarized component of 98.8%. Typically, the optical wedge 351 is fabricated with a 0.5 to 1.0 degree optical wedge which allows the beam from the front and rear optical surfaces to be spatially separated and prevents optical interference effects which can change the sampling ratio. The first surface reflection on line 355 is used for calibrated energy measurements. The second surface of the optical wedge 351 sometimes receives an anti-reflection coating to reduce the overall insertion loss to the main beam although typical A/R coatings often have reflectivities that are not much below the 0.6% of the uncoated surface for this case. A reflected component that is 0.6% of a 15-20 J pulse from a laser system used for example in laser peening, is still too energetic for a typical pyroelectric energy meter so the first optical wedge 351 is usually followed by a second optical wedge 352 to further attenuate the sampled beam, as shown in FIG. 1.
While this uncoated beam splitter method of FIG. 1 works for laser systems having well-polarized outputs, it does not work well when installed as an energy measurement system on systems involving the use of beam delivery optics that can cause small shifts in polarization. When a laser beam reflects from a mirror with a dielectric optical coating, the polarization is completely unchanged as long as the direction of polarization is in pure S-polarization or P-polarization, i.e. E-field perpendicular (for S-polarization) or parallel (for P-polarization) to the plane of incidence. However, if the beam has a non-orthogonal polarization, small depolarization errors can result, slightly changing the beam polarization and degrading the polarization contrast. As an arbitrarily oriented beam reflects from the various mirrors between the output of the laser system and the target, these depolarization errors accumulate. Although the total error might not be sufficient to degrade performance for a particular laser application, it can cause significant changes in reflectivity for a beam splitter using a double wedge pair as shown in FIG. 1. For example, a 2× increase (i.e. doubling) in light levels after two beam wedge surface reflections would result from a depolarization state that only changes the surface reflectivity by 0.25%. This is an almost inconsequential change in energy and polarization (a few degrees rotation) delivered to the target surface but causes a 2× error in measured energy.
It is desirable to provide systems that overcome one or more of the problems outlined above, including a beam splitter and a diagnostic system for high power systems that are polarization independent.